David Eagleman's previous book, Sum: Forty Tales from the Afterlives, was a delightful collection of short fables, each offering a wish-fulfillment image of life after death in which the wish turns out to contain its own perverse consequences. The fable principle was grounded in a nicely ironic psychology, subtly underpinned by Eagleman's own profession, neuroscience. Using fiction, Eagleman found a neat way of revealing how the mind cannot escape the contradictions of its underlying construction.

With this new book, Eagleman dispenses with fiction. This is a straight account of his own neuroscientific beliefs. Belief is the appropriate term, because Incognito isn't precisely an examination of neuroanatomy or neurological case histories; nor is it an exploration of the philosophical struggle involved in explaining the relationship between brain and mind. It is, rather, a breathless account of possible implications opened up by the rise of neuroscience as a way of looking at the world.

What are these implications? First, the process of learning more about the brain has changed our idea of what it means to be human. Man's sense of self has been rocked by key scientific revolutions in our understanding of the universe: the discovery that earth was not its centre, that time is deep not shallow, that humans were not God-created but a product of evolution. Brain science, Eagleman believes, provides the final frontier in our understanding of our own littleness and contingency: the realisation that consciousness is not the centre of the mind but a limited and ambivalent function in a vast cosmological circuitry of non-conscious neurological functions. Hence, most of our mental operations occur "incognito".

We should not worry about all this "decentering", Eagleman concludes, because science shows us that brain and mind and life are even more wondrous and exciting than we thought.

This interpretation of modern intellectual development is ahistorical and incorrect. As an enthusiast of Freudian models of the unconscious, it should be perfectly apparent to Eagleman that the decentering of the conscious mind took place long before the rise of contemporary neuroscience. We haven't needed fMRI scans, or software metaphors of brain circuitry, to tell us that we are subject to non-conscious drives that override our limited rational faculties. We got that much not only from Freud but from romantic poetry and 19th-century Russian novels.

Nor have we needed the finer developments of functional neuroanatomy to tell us that brain damage causes changes in behaviour, thus undermining simplistic notions of free will or criminal culpability. Eagleman canters through various well-known neurological cases, none original to this book, in which criminal acts or radical changes in personality have been shown to be the result of brain damage or disease. Appearing not to notice the glaring chronological anomaly, he cites the case of Phineas Gage, the American railroad foreman whose brain was violently punctured by an iron rod. Amazingly, Gage survived and could still function. But he was so drastically altered as a personality that colleagues could scarcely recognise him. The basic elements of the mind-brain problem have been chewed over in this case ever since it occurred – in 1848.

This book belongs to a popular trend of neuro-hubris – wildly overstating the ramifications of a science that is still in its infancy. The true fascination of neuroscience lies not in bombastic philosophical claims but in the fine detail of brain function, illustrations of the mind-brain problem, and the human interest of case histories. There isn't even that much actual neuroscience in Incognito. Its illustrations are drawn just as much from the annals of evolutionary psychology, behavioural economics and more traditional forms of psychology.

The contrast with Sum could not be more vivid. Eagleman is the rarest kind of science writer: better at translating his knowledge into fiction than explaining it as fact.